1. Field of the Invention
The present invention relates to the technological field of wireless communication, and more particularly, to a cell selection technique for determining which cell of which wireless communication system is to be accessed by a mobile terminal in the multi-system coexisting environment.
2. Description of the Related Art
Currently available mobile communication systems constituted by multiple cells include second-generation cellular phone systems using time division multiple access (TDMA) schemes, personal handy-phone systems (PHS), and third-generation cellular phone systems using code division multiple access (CDMA) schemes. So far, single-carrier CDMA scheme has become the mainstream in the third-generation cellular phone systems. When using single-carrier CDMA, the transmission bandwidth has to be broader as the transmission rate is increased, and the system becomes more susceptible to frequency selective fading. In addition, the chip rate is increased (that is, the chip length become shorter) due to the speed-up. Although the multipath separating ability is improved, the amplitude of each received path becomes small, and the degradation in RAKE combining becomes serious. For these reasons, certain limitation exists in speed-up in ordinary single-carrier CDMA. For the fourth-generation mobile communication systems aiming at the downlink transmission rate of 100 Mbps or more during high-speed travel, it is deemed that orthogonal frequency division multiplexing (OFDM) is the most promising candidate, in place of ordinary single-carrier CDMA, to achieve a high transmission rate. At present, OFDM is in digital terrestrial broadcasting, IEEE 802.11 wireless LAN, and HiperLAN.
A typical radio access scheme currently employed in the third-generation cellular phone systems is code division multiple access (CDMA), and the mainstream technologies are WCDMA and cdma 2000. these technology trends and the market needs have to be taken into account when introducing OFDM. In the 3GPP for discussing and establishing the spec of the 3G system employing a WCDMA scheme, it is discussed to make use of upper layers of a WCDMA system to introduce an OFDM scheme in the physical layer and the wireless domain. The contents of technology concluded and agreed by the 3GPP as to the introduction of OFDM in 3G systems are described in 3GPP documents. See 3GPP TR 25.892, V6.0.0 (2004-06).
One reason for introducing OFDM schemes in the WCDMA-based 3G radio access system is that the OFDM has a superior transmission characteristic even in the multipath propagation environment. Another reason is that OFDM can achieve higher throughput, as compared with WCDMA, when transmitting signals in parallel to the conventional high-speed downlink packet access (HSDPA) signals transmitted with fixed specs. It is supposed that in the hypothetical system used in the 3GPP discussion, the facilities of wireless base stations are shared by the OFDM system and the WCDMA system. Accordingly, the upper layers including the MAC layer of the current WCDMA system are shared by both systems, while physical layers are used by these systems independently from each other. In addition, the radio sections (such as an RF unit and an antenna unit) are shared by the two systems. Because, in such a hypothetical system, WCDMA signals and OFDM signals are transmitted from the same antenna, WCDMA cells and OFDM cells coexist, which different cells are formed as concentric layers.
The future (the fourth-generation) mobile communication systems are likely to employ multi-carrier signal transmission schemes, such as OFCDM (which is also called MC-CDMA) in which code spreading techniques are introduced in OFDM or OFDM-based schemes. Accordingly, a third-generation system and a fourth-generation system are likely to coexist in the course of transition from the third-generation system to the fourth-generation system and even after the fourth-generation systems become common. It is also likely that mobile terminal devices connectable to the both systems come up during the transition. The third-generation systems may continuously exist for the purpose of voice communications or low-rate data transmission even after the fourth-generation systems become widespread. In the WCDMA/OFDM coexisting system discussed at the 3GPP, mobile terminals are connected to a network using a WCDMA radio link, and accordingly, it is presumed that a mobile terminal using an OFDM radio link can also use a WCDMA radio link (i.e., connectable to a WCDMA cell).
As has been described above, a third-generation cells and a certain type of fourth-generation cell are formed in a concentric manner. This means that an overlapping area connectable to the both cell exists. The mobile terminal located in such an area has to select either cell to carry out radio communications. An OFDM cell strong in multipath interference will be suitable for high-quality and high-speed signal transmission, as compared with a WCDMA cell, as long as signal delay resides within the guard interval.
For the purpose of improving the tolerance to multipath interference, guard interval is inserted in an OFDM radio signal. If the time delay of a delayed wave is within the guard interval, the characteristic degradation due to delayed waves can be reduced greatly to substantially the negligible extent. This means that even if time delay of delayed wave components contained in the received at a mobile terminal is large, interference can be avoided by setting the guard interval longer. As the cell radius increases, time delay of the delayed wave is likely to increase because the signal propagation range increases. Accordingly, it is necessary to elongate the guard interval inserted in an OFDM signal to be transmitted in the wireless section of the cell. However, inserting long guard interval in a transmission signal will decrease the transmission efficiency because the guard interval itself does not contribute to data transmission. For this reason, it is desired for an OFDM cell to select a smaller cell radius to prevent degradation of the transmission efficiency as much as possible, and it is also desired to set the guard interval of the OFDM transmission signal as short as possible. It is naturally expected in the WCDMA/IFDN coexisting system that the OFDM cell size is set smaller than the WCDMA cell size.
A technique for optimizing the length of the guard interval according to the communication status is disclosed in, for example, JP 2002-345035.
If, in a WCDMA-cell/OFDM-cell coexisting system, a mobile terminal is compatible and connectable to both types of cells, to which cell the mobile terminal is to be connected has to be appropriately determined. It is expected from the above-described speculation that an OFDM cell will have higher performances (including transmission characteristic and throughput) in many cases under the situation where both types of cells are available.
In general, a pilot signal is used in a mobile communication system (including a third-generation cellular system) to select a cell from among multiple cells. The pilot signal is transmitted from each cell and received at a mobile terminal. A reception characteristic (such as a received SNR) is measured by the mobile terminal, and cell selection is carried out based on the measurement result. The load distribution (including the number of currently connected mobile terminals and the traffic) may be taken into account. Applicability of this method to cell selection in a WCDMA/OFDM coexisting system in which a WCDMA cell and an OFDM cell coexist as concentric layers is described in detail below. In the following speculation, it is assumed that the radium of the OFDM cell is set smaller than that of the WCDMA cell, and that a mobile terminal is located in the OFDM cell. In comparison between the power levels of the WCDMA pilot signal and the OFDM pilot signal, it is expected that the former one shows better receiving characteristic than the latter one.
As illustrated in FIG. 1, in order to guarantee the same receiving power level (Pth) at the cell boarders, the WCDMA pilot signal has to be transmitted with greater power as compared with the OFDM pilot signal. It is expected accordingly that the receiving power level (PCDMA) of the WCDMA pilot signal generally becomes higher than the receiving power level (POFDM) of the OFDM pilot signal when it is received at the mobile terminal located in the OFDM cell. As a result, the WCDMA cell is likely to be selected in spite of the fact that the mobile terminal is located in the OFDM cell with better signal transmission characteristic.
To overcome this inconvenience, it may be proposed to give an offset to the receiving power level of the pilot signals received at the mobile terminal to compensate for the difference in the receiving power levels of the two pilot signals. However, such correction to the receiving power levels is not easy. Even if such correction can be realized, it is still difficult to appropriately determine which cell is to be selected if the pilot signals from the both cells have the same received power levels.
This problem may occur not only in the WCDMA/OFDM coexisting system, but also in an arbitrary system in which different types of cells coexist.